Effect of Cumin Intercropping Density on Cotton Growth and System Economic Benefits under Subsurface Drip Irrigation

The mulch-free subsurface drip irrigation system demonstrated water-saving potential as an alternative to traditional mulch-based drip irrigation, while also eliminating residual film pollution at the source. However, delayed sowing is unavoidable in mulch-free cultivation within ecological regions with a short frost-free period. Intercropping with cumin, which has a shorter growth period, served as an effective strategy to improve land use efficiency during the early growth stages of cotton. Therefore, a two-year field experiment was conducted to study the effects of intercropping cumin at the seeding rate of 2.5 (ID1), 3.85 (ID2), and 5.2 (ID3) kg hm ^− 2 on cotton growth, interspecies competition, fiber quality and water use efficiency (WUE), as well as system economic benefits under subsurface drip irrigation. Monocropped cotton was used as the control (CK) treatment. The results indicated that at the initial flowering (IF) stage (the end of the co-growth period of cotton and cumin), ID2 and ID3 treatments decreased cotton plant height by 5.93–16.53% and 10.87–31.11%, respectively, cotton stem diameter by 11.41–14.25% and 3.37–26.49%, respectively, vegetative biomass by 14.46–30.65% and 22.59–49.91%, respectively, compared with CK treatment. With the increasing of cumin density, the crop growth rate (CGR) and compensation effect in cotton tended to significantly decrease at IF stage regardless of organs considered. For non-co-growth period (after harvesting cumin), ID2 and ID3 treatments increased cotton reproductive organ biomass by 4.09%‒14.61% at boll opening stage, crop growth rate in reproductive organs by 20.74% and 74.26% from peak boll to boll opening stages compared with CK treatment, due to an enhancement of 19.09% and 49.30% in compensation effect. Compared with ID1, the aggressivity treated by ID2 and ID3 decreased by 12.82–46.34% and 17.95–31.71%, respectively. However, owing to a greater number of green bolls in the upper canopy at the harvest stages in ID3 treatment, the system production value (closely related to yield) treated by ID2 was 11.69–16.89%, 6.56–20.02%, and 16.48–59.83% greater than that of ID1, ID3, and CK treatments, respectively. This also led to the highest WUE and net profit under ID2 treatment. In conclusion, intercropping cumin with medium-density improved the cotton biomass accumulation characteristics and increased resources such as land and water utilization efficiency and economic benefits through a stronger compensation effect after harvesting cumin under subsurface drip irrigation without mulch. This study not only provides alternatives to residual film pollution in arid cotton fields but also establishes a sustainable agro-ecological-economic planting paradigm by reducing plastic use and enhancing water and fertilizer use efficiency, holding significant implications for advancing resource-efficient agricultural systems.